1. Field of the Invention
The invention relates to a pixel unit of an organic light emitting diode and a display panel using such pixel unit.
2. Description of Related Art
Referring to FIG. 1, FIG. 1 is a diagram illustrating a conventional pixel unit 100 of organic light emitting diode (OLED). An operation principle of the pixel unit 100 is approximately as follows. When a switch SW1 formed by a thin-film transistor is turned on in response to a control signal CTRL, a data signal VDATA is stored in a capacitor CC. Moreover, when the switch SW1 formed by the thin-film transistor is turned off in response to the control signal CTRL, a driving transistor DTFT formed by a thin-film transistor is turned on, and correspondingly generates a driving current according to a voltage stored in the capacitor CC, so as to drive an OLED OD1.
A following equation (1) represents a driving current I flowing through the OLED OD1:
                    I        =                              1            2                    ⁢                                    K              ⁡                              (                                  VGS                  -                  VTH                                )                                      2                                              Equation        ⁢                                  ⁢                  (          1          )                    
Where, k is a process parameter (constant) of the driving transistor DTFT, VGS is a voltage between a gate and a source of the driving transistor DTFT, and VTH is a threshold voltage of the driving transistor DTFT.
Since the threshold voltage VTH of the driving transistor DTFT can be shifted due to long time positive bias driving, when the threshold voltage VTH of the driving transistor DTFT is shifted, a magnitude of the driving current is directly influenced. Therefore, a brightness of the OLED OD1 is changed and is hard to be controlled. In order to achieve a stable brightness of the OLED OD1, it is an important issue to be developed to effectively compensate the shifting phenomenon of the threshold voltage VTH.